How to get a pixelated image?
You can use a Pixelate node in the compositor. Go to the Compositing Screen Layout or open a Node Editor some other way. Make sure the Compositing Nodes button is selected.



Use the following setup. Notice how the left side of the gun is pixelated, but the right side is normal. In the final render, it will all be pixelated. Additionally, see how the both Scale node's X and Y values are equal. Also notice how between them they equal 1 (0.2 x 5 = 1). This means that the overall scale of the gun image doesn't change.



If you so desire, you can copy the node setup straight from the following file (gun not included) (sorry) (it does include an awesome hippie monkey, tho). Lastly, you can read more about the Pixelate node here.



Pixelated:



Normal: